A single-focus lens and a zoom lens widely used for photographing and broadcasting are generally constituted by a large number of lenses, for example, about 10 to 40 lenses.
Increase in the number of lens leads to increase in the total amount of light reflected by lens surfaces, and entering of repeatedly reflected light into a photosensitive surface, causing problems such as flare and ghost, which extremely deteriorate the optical characteristics. Accordingly, these lenses have multi-layer anti-reflection coatings comprising dielectric layers having different refractive indices, each dielectric layers having optical thickness of ½λ or ¼λ to a center wavelength λ for utilizing interference effects.
For example, JP 2007-213021 A discloses an anti-reflection coating comprising first to eighth layers laminated in this order on a substrate, the first and fourth layers being made of a low-refractive-index material having a refractive index of 1.35-1.50 to a d-line having a wavelength of 587.56 nm, the third and fifth layers being made of an intermediate-refractive-index material having a refractive index of 1.55-1.85 to the d-line, and the second and sixth layers being made of a high-refractive-index material having a refractive index of 1.70-2.50, which is higher than that of the intermediate-refractive-index material, to the d-line. This anti-reflection coating has reflectance of about 0.15% or less in a wavelength range of about 400-700 nm.
JP 2002-267801 A discloses an anti-reflection coating comprising first to ninth thin, dielectric layers laminated in this order on a substrate, the second, fourth, sixth and eighth layers having a refractive index of Nh at a wavelength of 550 nm, the first, third, fifth and seventh layers having a refractive index of Nm at a wavelength of 550 nm, and the ninth layer having a refractive index of Nl at a wavelength 550 nm, meeting 2.00≦Nh≦2.20, 1.50≦Nm≦1.80, and Nl≦1.46. This anti-reflection coating has reflectance of about 0.2% or less in a wavelength range of about 400-680 nm.
JP 2002-107506 A discloses an anti-reflection coating having a design wavelength λ0 of 550 nm and comprising 10 layers laminated on a substrate, the second, fourth, sixth and ninth layers having a refractive index of 2.00 or more at the design wavelength λ0, the first and seventh layers having a refractive index of 1.50-1.80 at the design wavelength λ0, and the third, fifth, eighth and tenth layers having a refractive index of 1.46 or less at the design wavelength λ0. This anti-reflection coating has reflectance of about 0.2% or less in a wavelength range of about 400-710 nm.
JP 2001-100002 A discloses an anti-reflection coating comprising first to tenth thin, dielectric layers formed in this order on a substrate, the second, fourth, sixth and ninth layers having a refractive index of Nh at a wavelength of 550 nm, the first and eighth layers having a refractive index of Nm at a wavelength of 550 nm, the third, fifth, seventh and tenth layers having a refractive index of NL at a wavelength of 550 nm, meeting 2.0≦Nh, 1.5≦Nm≦1.8, and NL≦1.46. This anti-reflection coating has reflectance of about 0.2% or less in a wavelength range of about 410-690 nm.
However, these anti-reflection coatings have as narrow reflection-preventing ranges as about 300 nm in a wavelength range of 380-780 nm, which is generally called visible band. Human eyes have strong sensitivity particularly in a wavelength range of 390-720 nm in this visible band.
JP 2000-111702 A discloses a 14-layer anti-reflection coating comprising layers having a refractive index of 1.6 or more and layers having a refractive index of 1.5 or less for having wide-band reflection characteristics providing reflectance of 1% or less in a wavelength of 330-710 nm, and reflectance of 0.25% or less in a wavelength of 400-680 nm. However, this anti-reflection coating has reflectance of at most 0.25% or less in a reflection-preventing band, particularly in a visible band, failing to meet the recent demand of visible reflectance of 0.2% in digital camera lenses.
JP 2002-14203 A discloses an anti-reflection coating comprising 14-17 layers formed by TiO2 having a refractive index of 2.407 and SiO2 having a refractive index of 1.450 for having reflectance of 0.1% or less in a wavelength of 400-700 nm. However, the reflection-preventing wavelength range of this anti-reflection coating is as narrow as 300 nm in a wavelength range of 380-780 nm, which is generally called visible band.